Aircraft speed measuring system designed to avoid propeller modulation effects



3 Sheets-Sheet 2 Zinneutor L'ino LiMar'l'inelli Quorum NNN Y c. c. MARTINELLI AIRCRAFT SPEED MEASURING SYSTEM DESIGNED TO AVOID PROPELLER monumnou EFFECTS mbwk m Jan. 9, 1951 Filed Jan. 29, 1947 Jan. 9, 1951 c. c. MARTINELLI 2,537,596

AIRCRAFT SPEED MEASURING SYSTEM DESIGNED TO AVOID PROPELLER MODULATION EFFECTS Filed Jan. 29, 1947 3 Sheets-Sheet 3 CORRECT SPEED 6 R50 wums 6km! PQWER Sw/ TCI'H '85? Ma R544) 1 39 Zhmcutor Cir-b LZMa rl'in'elli Gttorneg Patented Jon. 9, 1951 AIRCRAFT SPEED MEASURING SYSTEM DE- SIGNED TO AVOID PBOPEILEB MODULA- TION EFFECTS Ciro C. Martinelli, Princeton, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application January 29,1947, Serial No. 724,995

'1 Claims. on. 343-8) My invention relates to radio apparatus that utilizes a reflected radio signal and the Doppler eifect for determining relative speeds and partlcularly to apparatus for determining the speed of approach of an aircraft about to land. In designing such equipment it has been found that the presence of propeller modulation on the reflected signal makes it necessary to take special precautions in the equipment design if successful operation is to be obtained.

For example, the specific apparatus described hereinafter transmits at a frequency of 1500 megacycles per second. It is designed for aiding in the landing of aircraft on aircraft carriers where the relative speed of the aircraft and the carrier may be as low as 40 knots. Under these conditions the Doppler beat frequency will be as low as 200 cycles per second. Certain types of carrier type aircraft have four-bladed propellers and when making carrier approaches may have rotational speeds as high as 2500 R. P. M. with a resulting propeller modulation frequency of apprommately 160 cycles per second. It has been observed, however, that generally the propeller modulation frequency is from 60 to 90 cycles per second. This propeller modulation is both amplitude and frequency modulation of the Doppler frequency signal.

Since the Doppler frequency is a function of the closing speed of an aircraft, i. e., of the relatlve speed of the aircraft and the aircraft carrier, fixed filters cannot be used to obtain the desired separation of the Doppler frequency and the propeller modulation frequency. It will be apparent that with the Doppler frequencies and the propeller modulation frequencies so close together it is a problem to separate them over a range of Doppler frequencies. The range of Doppler'frequencies usually is from about 250 cycles per second to 350 cycles per second, corresponding to speeds of from 50 knots to 70 knots.

The problem is further complicated by the fact that the signal due to propeller modulation is not a definite fixed frequency but, instead may be represented by sidebands of the Doppler frequency signal with all components fluctuating at a fairly fast rate. The signal may fluctuate from a condition where most of the energy is in one side band to one where most of the signal is in the other side band.

An object of the invention is to provide an improved method of and means for determining the speed of approach of an aircraft.

A further object of the invention is to provide an improved method of and means for determining the relative speed of an aircraft and an aircraft carrier.

A still further object of the invention is to provide in a relative speed determining system an improved method of and means for separating propeller modulation currents from Doppler frequency currents.

A still further object of the invention is to provide an improved speed determining radio system of the type utilizing reflected radio waves.

According to a preferred embodiment of the present invention, an unmodulated radio wave is transmitted toward the approaching aircraft and the reflected signal is mixed at the receiver with signal direct from the transmitter. The Doppler signal is then obtained by demodulating the signal thus mixed. The resulting demodulated signal contains not only the desired Doppler signal but also a group of signals produced by the rotating propeller of the aircraft as previously explained. This mixture of signals is amplified and passed through a limiter to a frequency counter which is connected in a servomotor circuit so that the motor shaft assumes an angular position that is a function of the frequency measured by the counter.

The servo system is given a time constant that is slow compared with the frequency fluctuations introduced by the propeller modulation. The result is that the servomotor shaft assumes an angular position representative of the mean or average frequency of signals applied to the frequency counter. In operation the servomotor shaft will be continually in motion, but although the motion will be within limits representing a narrow frequency range this shaft cannot be used to provide an accurate reading of the closing speed. Instead this shaft is connected to a frequency selective circuit to tune it automatically to the Doppler frequency that is obtained as an aircraft comes in to land. Some of the signal from the limiter preceding the counter is supplied to the selective circuit whereby its output signal is the Doppler frequency signal substantially free of propeller modulation signal.

This Doppler frequency signal is then passed through another limiter and supplied to another frequency counter which is connected in a servo system for providing a direct indication of the closing speed and for operating speed indicator lights.

The invention will he better understood from the following description taken in connection with so the accom drawing in which ass-7,596

Figure 1 is a block diagram of one embodiment of the invention,

Figures 2 and 3 placed end to end are a circult diagram of the portion of the circuit in Fig. 1 shown at the right of the broken line.

Figure 4 is a view of the mechanical arrangement of a portion of the apparatus shown in Fig. 3.

In the several figures similar parts are indicated by similar reference characters.

Fig. 1 shows an embodiment of the invention comprising a radio transmitter that supplies an unmodulated carrier wave to a directive antenna H from which the carrier wave is radiated toward the aircraft approaching for a landing. After reflection from the aircraft, the carrier wave is picked up by a receiving antenna |2 and supplied to a mixer or first detector l3 of a superheterodyne type receiver.

The local het-rodyning signal isobtained from a local oscillator |4 operating at the desired intermediate frequency which, in the present example, is 120 megacycles per second. Signal from the oscillator l4 and signal from the transmitter ID are supplied to a mixer l6. Assuming a transmitter frequency of 1500 megacycles per second, the resulting mixer output consists of the 1500 mc. carrier and the upper and lower sidebands of 1620 me. and 1380 mc., respectively. The mixer output is supplied to a side band filter I! which passes only the lower side band signal of 1380 me. and supplies it to the mixer I3 where it mixes with the reflected signal of 1500 me. plus Doppler frequency where the aircraft is approaching. The output of mixer l3 comprises the I.-F. carrier of 120 me. plus Doppler frequency. This signal is amplified by an L-F. amplifier l8 and supplied to a second detector I9 where it beats with 120 me. signal supplied from the local oscillator M.

The output of the detector I9 is the desired Doppler frequency signal which indicates the speed of the approaching aircraft but, in addition, the output includes undesired propeller modulation signal as previously explained. This output is amplified by an audio frequency amplifier 2| which may be designed to pass a band of frequencies of from 200 to 700 cycles per second. Cut-off at the lower frequencies is desirable to reduce the effects of sea-return signals and cut-off at the higher frequencies is desirable to reduce noise signals.

The output of amplifier 2| is supplied through an amplitud limiter 22 to a frequency counter 23. The counter 23 controls a servomotor system 24 to drive a shaft 26 in one direction or the other as a function of the frequency of the signal supplied to the counter 23. The shaft 26 controls the tuning of a selective circuit 21 to keep it tuned approximately to the Doppler frequency. Signal from the limiter 22 is supplied over conductors 28 to the selective circuit 21 whereby an output is obtained that is free of most of the propeller modulation interference. This output is supplied over conductors 29 to an amplitude limiter 3|.

The output of limiter 3| is applied to a frequency counter 32 the output of which controls a servomotor system 33 which drives a shaft 34 in one direction or the other as a function of the Doppler frequency measured by the counterment 3? of colored lights which indicate whether a the closing speed is correct, too slow or too fast.

The system is provided'with an automatic gain control or AVC circuit 38 and also with a hold-off circuit 39 for holding the counter and servo units in an unchanged condition when the incoming signal fades out. Thus the system can pick up its proper control action as soon as the signal comes back to its normal strength.

The details of the portion of the system at the right of the broken line in Fig. 1 are shown in Figs. 2 and 3. Referring to these figures, the AVG circuit compris s a diode 4| that has its cathode connected through a resistor 42 to a point on a potentiometer resistor 43 by way of a variable tap 44. Adjustment of the tap 44 sets the threshold or delay voltage. The anode of diode 4| is connected through an output resistor 46 to the grounded end of the potentiometer resistor 43. AVG voltage is taken off the anode end of resistor 46 and supplied through a filter 41 to the L-F. amplifier l8 (Fig. 1).

A hold-01f circuit comprises a diode 40 that has its cathode connected to the cathode and of resistor 42 so that the diode 48 receives signal from the audio amplifier 2| and so that the resistor 42 provides the direct-current return path for diode 48 as well as for diode 4|. The anode of diode 48 is connected through an output resistor 45 to ground and is direct-current coupled to the grid of a triode 49. The anode of triode 49 is direct-current coupled to the grid of .a triod 5|. The anode circuit of triode 5| includes the coil 50 of a relay K204 so that the armature 52 of this relay is on the lower contact point when the coil 50 is energized, thus energizing a relay, K405 (Fig. 3). Another armature of the relay K204 which is also actuated by the coil 50 but shown at a different point in the drawing is provided for a purpose described hereinafter.

The relay K204 is energized so long as the received signal is of sufficient strength and is deenergized when the signal fades away. The operation is as follows: Signal from the amplifier 2|, when of usable amplitude, produces current flow through the diode 48 and the output resistor 45 thus making the anode end of resistor 45 negative and biasing the tube 49 to plate current cutoff. This makes the grid of the tube 5| more positive whereby the resulting plate current flow energizes the relay K204. It may be noted that even though the current flow through the resistor 42 is in the direction to keep the diode 48 from conducting, the applied signal causes diode 48 to conduct because its amplitude is about twice that of the voltage drop in the resistor 42, the resistors 42 and 46 being of about the same resistance.

The AVC circuit associated with the diode 4| has a slow time constant which is required for good AVC action, while the circuit associated with the hold-01f diode 48 has a fast time constant so that the counter and servo system is promptly put under control of the received signal as soon as it comes back to usable strength after a fade. The control action of the hold-off circuit will be discussed more in detail hereinafter.

Referring now to the counter and servo system shown in Fig. 2, the counter 23 comprises a pair of oppositely connected diodes in a common envelope 52 to which the square wave from limiter 22 is applied through a capacitor 53 of comparatively small capacity. Storage capacitors 54 and 56 of comparatively large capacity have a charge supplied to them through the cathode 51 and the anode 08 of one diode upon the occurrence of each negative half cycle of the square is proportional to the frequency of the limiter output signal.

The diode comprising an anode 59 and a cathode BI is provided to discharge the capacitor at the end of each negative half cycle to prevent it from blocking. The cathode GI is connected to a positive bias when the relay K205 is energized as illustrated, i. e., when there is a usable incoming signal, assuming that the relay K201 is not energized. The relay K201 is never energized unless the switch 1i shown in Fig. 3 is set on the "air speed preset" position. This connection to the positive bias voltage is through the armature 02 of relay K205, through the armature 03 of relay K201 and through a lead 64 and an adjustable tap 86 to a point on a potentiometer resistor 61.

The anode 58 is connected through a resistor 08 to the armature 69 of the relay K201 but this circuit is open unless the switch 1I (Fig. 3) is on the air speed preset position. In the drawing all relay and switch positions are shown for the condition of automatic operation for tuning the Q trated. This connects the motor 10 to the 28 volt source to make it run in one direction. when the relay K202 is deenergized, the relay K203 is likewisedeenergized, the armatures 0| and drop down, and the motor 10 runs in the opposite direction.

The shaft 26 is coupled, as indicated by the broken line 83, to three variable taps on resistors 04, 06 and 01 of the twin-T frequency selective circuit 21 so' that the tuning of this circuit is a function of the angular position of'the shaft 28. As a result, substantially only the Doppler frequency component of the signal applied to the selective circuit 21 automatically and for the condition of an incoming signal of usable strength.

In the present example, the counter storage capacitors 54 and 56 are the shunt elements of a resistor capacitor filter having a series resistor element 12 so that a fairly long electrical time constant is provided. The low potential sides of the capacitors 54 and 56 are connected to ground through the armature 13 of the relay K205 unless the signal fades out, in which case the circuit to ground is opened and the capacitors hold the charge they had at the moment of signal fad- 111 The diode anode 58 is also connected through a resistor 14 and a lead'16 to a point of positive potential on a potentiometer resistor 11. This is a servo connection and changes the positive bias potential on the anode 58 as the servomotor 18 operates to bring the frequency counter to a balanced condition. The motor 18 is reversible and through reduction gears 19 drives the tap 8| on potentiometer 11 in one direction or the other to short out varying amounts of the,pctentiometer resistance thereby varying the potential applied to the lead it. v

The high potential and of the storage filter M, 56, 12 is connected through an isolating resistor82 and through the secondary 83 of atransformer 84 to the grid of a relay amplifier tube at. A derivative or anticipator capacitor 01 is connected between the lead 16 and the grid end of resistor 82 to provide suitable damping for the servo system.

The amplifier tube 06 has an operating voltage applied to its anode through a plate resistor 81. The cathode of the tube 86 is connected to a positive bias potential on the resistor 81.

The anode of the tube 86 is direct-current coupled to the grid of a cathode follower tube 88 which has the coil 09 of a relay K202 in its cathode circuit. When the relay K202 is energized,

the relay armature .89 is in the position shown to energize the relay K203 and pull the relay arselective circuit 21 by way of the lead 28 appears on the output lead 29.- Before describing the selective circuit" in further detail, certain features of the servo system 24 will be discussed.

In order to control hunting of the, servo system, the motor 18 is made to oscillate about the balance point by introducing a ripple voltage on the grid of the relay amplifier 86. This is done in the present example by supplying a sawtooth wave to the transformer 84 from a generator 90. The generator 98 is a relaxation oscillator comprising a capacitor 99 that is shunted by a gas tube IOI. A charging voltage is applied to the capacitor 89 through a resistor I02 so long as the armature I03 of the relay K204 is closed as is the case except when the signal fades out.

The capacitor 99 charges and discharges periodically through the gas tube IM to produce a sawtooth wave which is applied through a resistor I04 to the primary of the transformer 84'. The secondary 83 preferably is tuned to the repetition rate of thesawtooth wave so that substantially a sine wave is impressed upon the grid of the tube 86.

The overall time constant of the counter 23 and servo-motor system 24 is rather long and may be three or four seconds for example. This causes the counter and servo system to operate on an average of the applied frequency components, theaverage being taken over a period long enough. to prevent a strong frequency component due to propeller modulation and of short duration from affecting the angular position of the servo motor shaft 26 any great amount. At the same time, this time constant is not so long as to prevent the servo system from following changes in speed of the approaching aircraft.

In the apparatus illustrated, the above-mentioned overall time constant is provided about equally by the storage filter 54, 56, 12 and by the reduction gearing 19. It will be apparent that most of the desired time constant can be provided at 19 if desired by using a greater gear reduction ratio.

Referring now in more detail to the twin-T selective circuit 21, it comprises an amplifier tube I06 having an anode I01, a grid I08 and a cathode I09. Operating voltage is applied to the anode I 01 through an anode resistor I I i. A positive bias voltage is applied to the grid I08 from the junction point of potentiometer resistor H2 and H3. I

The cathode I09 is connected to ground through the anode-cathode impedance of a vacuum tube I It and through a cathode bias resistor H6 for the tube H4. The resistor H6 is bypassed for audio frequency signals by a capacitor II1.

The anode of the tube I06 is direct-current connected to the grid H8 of a cathode follower tube II9 having a cathode output resistor I2I.

The desired frequency selection is obtained by a degenerative feed-back connection from the cathode of the tube M9 to the grid of the tube II4 through a twin-T filter which includes the variable resistor elements 94, 96 and 91. The twin-T circuit will pass all frequencies except the Doppler frequency to which it is kept tuned bythe servo system 24 during automatic operation of the equipment. Thus, the selective circuit is highly degenerative for all frequencies but the Doppler frequency whereby substantially only the Doppler frequency is supplied to the output lead 29.

- The twin-T filter is of a well known type comprising the series variable resistor 94 and 93 of the same resistance value R0, the series fixed resistors I22 and I23 of the same resistance value R1. a shunt capacitor I24 of capacity 20, bridging capacitors I26 and I21 each of capacity C, and a resistance arm from the junction point of capacitors I26 and I21 to ground comprising the variable resistor 91 of resistance and the fixed resistor I28 of resistance The resonant frequency f of this filter (at which there is a transmission null) is iven by the equation It can be shown that for proper tracking of the resonant frequency of the selective circuit 21 with the input frequency to the counter 23 the ratio of R to R1 must always be equal to the ratio of the follow-up potentiometer resistor values R2 to Rs where R2 is the resistance of potentiometer resistor 8| and R: is the sum of the resistances of potentiometer resistors 11 and 11a. This condition can readily be obtained by using linear resistors of the proper values and by providing a screw driver adjustment for the resistor 11a.

Referring to Fig. 3, the Doppler frequency signal, now substantially free of propeller modulation, is supplied by way of the lead 29 and the limiter 3| to the frequency counter 32 which is or the other by a reversible motor I63 through reduction gears I54 and the shaft 34. The shaft 34 assumes an angular position that is a function of Doppler frequency, and therefore, of the closing speed which may beindicated by a pointer 36 connected to the shaft 34 and associated with a dial calibrated in knots. The shaft 34 is also coupled to a pair of cams I66 and I51 for operating the speed indicator lights I63, I63 and I6I as described hereinafter.

Referring now to the servo relay operation for controlling the servomotor I53, the servo relay comprises a second cathode follower tube I62 having a cathode output resistor I63 and having its grid connected to a point of positive bias poten- 'tial. Connected between the cathode ends of resimilar to the counter 23 but provided with a comparatively short time constant. The counter 32 comprises a pair of oppositely connected diodes in a common envelope I3I. Upon the occurrence of each negative half cycle of the square wave output from the limiter 3I a pulse of current passes through the diode having elements I 32 and I33 to apply a charge to the storage filter comprising elements I34, I36 and I31. The high potential side of filter I34, I36, I31 is connected through an isolating resistor I33 to the grid of a cathode follower tube I39 having a cathode output resistor MI.

The other diode of the counter 32 has its cathode I42 connected to a positive bias point at the cathode end of resistor I4I.

The anode I33 of the counter diode I 32, I33 is also connected through a resistor I43, a follow-mp lead I44 and a, variable tap I46 to a point of positive bias potential on the resistor I41 of a follow-up potentiometer. This follow-up potentiometer comprises the resistor I41, a resistor I43 and two resistors I49 and I5I which are adjustable for calibrating purposes. In any Doppler frequency range it is onLvnecessary to adJust sistors I4I and I63 is a relay coil I64 of a polarized relay for actuating a relay armature I66. The direction of current flow through the coil I64 depends upon whether the counter output voltage on the grid of tube I33 brings the cathode of this tube above or below the fixed potential of the cathode of the tube I62. When the relay coil current flows in one direction the armature I66 pulls over to the right so that the polarity reversing relay K402 is energized by the 28 volt source I61 providing the power switch I63 and the armature I69 of the relay K405 are in their closed positions. The armature I69 will be in the closed position illustrated providing va usable signal is being received since the relay K406 is energized so long'as the hold-off relay K204 is energized.

When the reversing relay K402 is energized, the relay armatures HI and I12 are pulled up and the motor I53 is driven in one direction. When the current through the relay coil I64 is reversed, the relay K402 is deenergized and the armatures HI and I12 go to their lower position thereby reversing the direction of rotation of the motor I53.

In the servo system 33 huntin is minimized by providing a vibrating action for the relay I64, I63 instead of by superimposing an alternatingcurrent component as in the servo system 24. This vibrating action is obtained by means of a coil I13 that is wound on the coil I64 and connected through leads I14 and I16 to the input leads of the motor I53. Each time the current flow through the coil I64 moves the armature I63 in one'direction, the resulting current flow to the motor I63 energizes the coil I13 by way of leads I14 and I16 so as to pull the armature I66 back in the other direction. Thus the relay I64, I66 vibrates at a rate that is largely determined by an inductance coil I11 that is connected in series with one of the leads I14 and I16. The rate of vibration may be about 20 times per second.

The overall time constant of the servo system 33 is fast as compared with that of the servo system 24 so that the speed indication follows closely any changes in the closing rate.

Referring in more detail to the cam control of the speed indicator lamps I53, I59 and I6I, the cams I56 and I51 actuate switch arms I13 and I19, respectively, so that as the two cams lead Ill supplying 28 volts to the lamps. There are 28 volts on the lead Ill for operating the lights if the switch arm Ill, which is ganged with switch arm 'II, is in either the-fautomatic position shown or in the air speed'prese position,

provided the power switch and the relay armature Ill are closed. .The indicator lights may be cut off by moving the switches Ill and III to their middle position.

If the closing speed is increased to the correct value, theservo shaft ll rotates the cams Ill and Ill counter-clockwise sufliciently to bring the switch arm I'll in contact with he lower contact point, switch arm I 18 remaining on its lower contact point. Thisllghts the white lamp "9 only, the high side of the lamp filament being connected through the lead Ill, the lower contact point of the switch arm I'll, the arm I'll, the lead Ill, and through the lower contact point of switch arm I'll and the arm I'll to the lead Ill.

If the closing speed is too fast, the servo shaft ll rotates the cams counter-clockwise sufliciently to bring the switch arm Ill in contact with its upper contact point, the switch arm I'll still being in contact with the lower contact point. This lights the green lamp I GI, the circuit from the high side of the lamp filament being through the lead I88, the switch arm I'll, the lead Ill, and through the switch arm I'll to the lead Ill.

It may be desirable at times to operate the 7 system by setting the selective circuit 21 to a fixed Doppler frequency so that the indicator servo 33 is operated by applied signal only when an aircraft is approaching at the correct closing speed. Thus. in the pre ent exam le there will he no operation of the indicator lights except when the white light I5! lights up to indicate that the closing speed is correct. To operate the system in this manner, the switches II and Ill are moved to the air speed prese position. This energized the relay K291 (Fig. 2) so as to connect the "anode 58 of the counter. 23 to a lead Ill, and through the lead I89 to a variable tap I9I on the resistor Ill! of a potentiometer I92. By changing the position of the tap I9I, an operator can change. the angular position of the servo shaft 26 and, therefore, can adjust the tuning of the selective circuit 21. The potentiometer I92 includes a fixed resistor I 95 and also variable resistors I93, IM and I96 which are used to adjust the current through the potentiometer and to adiust its voltage with respect to ground so that the frequency selective circuit 21 tracks with the air speed pointer I9! coupled to the tap ISI.

It will be noted that when the relay K201 is energized the cathode ll of the counter 23 is disconnectedfrom its positive bias supply so that the counter blocks and does not supply any sig- 119.1 to the servo system ll.

If the system is on automatic operation and the signal fades away, the counter 23 is blocked by the armature $2 of relay K205 being released to the open circuit position. During the raging 10 period of low signal strength, the charge on capacitors ll and ll is prevented from leaking oflbecausethearmature'lloftherelayxlll is also to its open circuit position. At the same time the ripple voltage is taken of! the servo relay since the armature Ill of relay Klll is in its open circuit position thereby taking voltage oi! the oscillator ll. It will be apparent that as soon as the signal comes back to usable strength the operation of the servo system begins at the point where the operation had been stopped by the fadingl adingawayofthesignalalsoresultsindeenergization of the relay Klli (Fig. 3) and opening of the relay armature Ill so that the 28 volt power supply to the servomotor Ill is broken. Thus the speed indication during a fadingintervalremainsthesameasitwasat the time the last usable signal was being received.

A "wind over carrier" control is provided which is indicated in Fig. 3 and shown in more detail in Fig. l. The fimction of this control is to add electrically to the follow-up voltage of the indicator servo a voltage which is proportional to the component of wind over the carrier which is in the direction opposite to the course of the aircraft. Actuallv, the control is the follow-up control potentiometer Ill which is mechanically mounted on the servo shaft ll so that while the shaft rotates the brush I ll of the potentiometer, the "wind 'over carrier control rotates the stator Ill or case of the otentiometer. This is shown more clearly in Figure 4. For servo system balance, the follow-up voltage at the tap Ill must eoual the voltage developed across the freouenc counter resistor I ll. If the stator ill of the potentiometer is moved with re pect to the brush Ill the servo shaft ll rotates the brush so that the resultant follow-u voltage is still selective circuit 21 must still be set to closing rate althou h with preset o eration the airs eed control is set for the desired airs eed of the aircraft. Because of this it is necessarv to subtract from the airspeed control a voltage eouivalent to the wind. Th s is done by linkin the stator I of the airspeed control potentiometer to the wind over carrier" control. This me han cal connection is shown in Figure 4 as a shaft 208. Werethis not done, the selective circuit 21 would. under reset o eration, be set to airspeed instead of closing rate.

I claim as my invention:

1. In a radio svstem for measuring relative speed by refle ted radio waves. radio transmitter means for transmitting radio waves toward a reflecting object. radio receiver means for receiving said waves after reflection from said object means for deriving from said received signal a signal having a fre uency that is a function of said relative speed, said derived signal having undesired rapidly fluctuating side band components, a filter tuning control means said control means having a time constant that is long compared with the rate of said fluctuations, means for supplying said derived signal to said control means and for operating said control means thereby, a filter circuit that is tunable 11 to the frequency of, said derived signal, means for causing said control means to tune said filter circuit substantially to said frequency in response to operation of said control means by application thereto of said derived signal, a frequency indicator means, and means for supplying said derived signal through said filter circuit to said frequency indicator means.

2. In a radio system for measuring relative speed by reflected radio waves, radio transmitter means for transmitting radio waves toward a reflecting object, radio receiver means for receiving said waves after reflection from said object, means for mixing said received signal with signal derived from said transmitter for obtain:- ing a Doppler frequency signal, a servo system having a comparatively long constant with respect to fast variations in the received signal, means for supplying said Doppler frequency signal to said servo system and for operating said servo system thereby, a filter circuit that is tunable to said Doppler frequency, means for causing said servo system to tune said filter circuit substantially to said Doppler frequency in response to operation of said servo system by application thereto of said Doppler frequency signal, a frequency indicator means, and means for supplying said Doppler frequency signal through said filter circuit to said frequency indicator means.

3. In a radio system for measuring relative speed by reflected radio waves, radio transmitter means for transmitting radio waves toward a reflecting" object, radio receiver means for receiving said waves after reflection from said ob- Ject, means for mixing said received signal with signal derived from said transmitter for obtaining a Doppler frequency signal, said Doppler frequency signal having an undesired frequency modulation thereon which varies at a comparatively rapid rate, a servo system having a time constant that is long compared with said rapid rate, means for supplying said Doppler frequency signal to said servo system and for operating said servo system thereby, a filter circuit that is tunable to said Doppler frequency, means for causing said servo system to tune said filter circuit substantially to said Doppler frequency in response to operation of said servo system by application thereto of said Doppler frequency signal, a frequency indicator means, and means for supplying said Doppler frequency signal through said filter circuit to said frequency indicator means whereby the side band components resulting from said frequency modulation are substantially removed.

4. The invention according to claim 3 wherein said frequency indicator means comprises a frequency counter followed by a servo system having a comparatively short time constant.

5. In a radio system for measuring the relative speed of an aircraft and a reflecting surface by reflected radio waves, radio transmitter means for transmitting radio waves oward said airi2 craft, radio receiver means for receiving said waves after reflection from said aircraft, means for deriving from said received signal a signal having a frequency that is a function of said relative speed and that is unavoidably modulated by the propeller of said aircraft, a filter tuning control means said filter tuning control means having a time constant that is long compared with the rate of the fluctuations in the side band components of said propeller modulated signal, means for supplying said derived signal to said control means and for operating said control means thereby, a filter circuit that is tunable to the frequency of said derived signal, means for causing said control means to. tune said filter circuit substantially to said frequency in response to operation of said control means by application thereto of said derived signal, a frequency indicator means, and means for supplying said derived signal through said filter circuit to said frequency indicator means.

6. In a radio system for measuring the relative speed of an aircraft and a reflecting surface by reflected radio waves, radio transmitter means for transmitting a continuous radio wave toward said aircraft, radio receiver means for receiving said waves after reflection from said aircraft, means for mixing said received signal with signal derived from said transmitter for obtaining a Doppler frequency signal that is unavoidably modulated by the propeller of said aircraft, a servo system having a time constant that is long compared with fluctuations in said frequency modulation, means for supplying said Doppler frequency signal to said servo system and for operating said servo system thereby, a filter circuit that is tunable to said Doppler frequency, means for causing said servo system to tune said filter circuit substantially to said Doppler frequency in response to operation of said servo system by application thereto of said Doppler frequency signal, a frequency indicator means, and means for supplying said Doppler frequency signal through said filter circuit to said frequency indicator means.

7. The invention according to claim 6 wherein means is provided for rendering said servo system inactive in response to a decrease in the strength oi the received signal below a usable level.

CIRO C. MARTINELIJ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Sanders Dec. 16, 1947 

